1. Field of the Invention
The present invention relates to an antenna used mainly in mobile telecommunication, particularly to a compact antenna suited to be mounted on the shoulder of a human body.
2. Description of the Related Art
In recent years, demands for mobile communications using a radio unit such as a portable telephones have been remarkably increasing. Such a radio unit may comprise a compact and low profile type planar antenna. As a compact antenna for a portable telephone, the planar inverted F antenna has been used. Constitution of such an antenna is described in "Performance Analysis of a Built-in Planar Inverted F Antenna for 800 MHz Band Portable Radio Units", T. Taga and K. Tsunekawa, IEEE Trans., vol. SAC-5, No. 5, pp. 921-929 (1987) and in the Japanese Patent Application No. 2-250655. The planar inverted F antenna is compact in construction, and is capable of transmitting and receiving both vertically and horizontally polarized waves, and is therefore suitable for portable telephones used in a multiple propagation environment.
However, in the prior art described above, since the planar inverted F antenna is installed on the body case of the radio unit and the antenna is located close to the human body during the operation of the potable telephone, the gain of the antenna decreases significantly. Also since the planar inverted F antenna functions as an antenna when it is connected to the body case of a radio unit having a ground plane sufficiently large relative to the antenna, it has been impossible to install the antenna separately from the radio unit. As a result, there has been a limit to the size reduction of the radio unit, and it has been impossible to reduce the size of a radio unit to such an extent that it causes no trouble to the user at all to carry it. Thus the use of such a conventional radio unit which is not compact enough has been inconvenient, particularly for those engaged in jobs which require them to always carry the radio units with them.